Bonnets serve multiple purposes in modem vehicles. First of all, the non-perforated parts of a bonnet separate the inner engine bay from the environment to avoid unwanted dust or debris entering and also to guide the air inside the engine bay via the grids. Therefore, the bonnet must be partly sealed against the surrounding parts of the vehicle chassis, especially in the area between air entry grids and the cooling package to force the entering air to pass through the grids for filtering reasons.
Concerning the cooling system, the perforated grids integrated in the bonnet provide the air intake to the heat exchangers and serve as a sieve to retain debris exceeding a several size from soiling the heat exchangers of the cooling system which results in decreased cooling performance or damage to the cooling fan.
In addition to this, as styling requirements are concerned, the design of the motor bonnet has a major impact on the overall styling of a vehicle, especially where tractors are concerned.
It is well known to affix perforated grids to the bonnet. These grids are made of flat steel plates in which closely positioned apertures are stamped. These apertures may be of holohedral form being of round or hexagonal or other shapes of about 1-2 mm diameter and 0.5-2 mm distance apart. Afterwards, these plane grids are cut to the required outer shape and are pressed or deep-drawn to obtain a three-dimensional shape needed for stiffness and installation.
Due to the fact that the apertures are holohedral and the outer geometry is cut before the three-dimensional forming process, the edges of these grids are shaped like a saw which can easily injure operator and the outer geometry can be corrugated, which must be seen negative from the styling point of view. These corrugations are caused by varying flow characteristics of the material during deformation.
As an alternative, the perforation is not holohedral by providing a un-perforated area around the outer periphery of the grid. This requires the apertures in the flat steel plate to be stamped in a manner dependent on the required shape of the grind which is much more expensive to produce, reduces intake surface and does not prevent corrugations on the edges.
As an alternative, the outer shape could be finished after forming the grid. This requires an additional cycle in production increasing costs.
As an alternative, grids are covered with rubber material. This requires a thicker coating which reduces the size of the apertures which weakens the structure if the apertures are increased in size to offset the thicker coating. These rubber coatings are also more expensive than thin galvanic powder coatings or dip coatings, which are normally used for corrosion protection.
Due to this, these grids are mainly assembled to the inner side of the bonnet to cover the sharp and corrugated edges of the grid. This type of the assembly is sometimes prohibited by the shape of the mating contours. For example, the receiving part could be u-shaped with limbs reducing their distance towards the end. Therefore, especially for complex three-dimensional forming, the inserted part cannot be positioned inside or only by damaging the painting.
Concerning the assembly of grids to the bonnet an additional aspect must be considered. The stamping procedure produces apertures with little rims extending from the grid surface. In combination with the sharp edges of the outer shape of the grid, this increases the danger that the painting of the part which is to receive the grid is damaged when contacted by the grid. Even when good contact is provided, the lower stiffness of the grid causes movement relative to the stiffer receiving part due to vibration. This type of damage may be avoided by screwing the parts together in the contact areas. Circumferential screwing does increase the assembly efforts and increase the requirements on shape and position tolerances thus increasing manufacturing costs.
To overcome this problem, grids are positioned into the receiving parts by using adhesive and sealing means which must be applied manually and carefully and hardened afterwards during assembly thus increasing assembly time.
In general overlapping contours reduce the perforated surface of a grid and thereby reducing air intake surface. Thereby air intake capacity is reduced or higher air speed must be accepted which is increasing pressure drops and debris suction.